In wireless communications networks one of the most important and fundamental problems is to set the output power of the transmitting devices. In many systems, for example in Long Term Evolution, LTE, the radio links have a Downlink, DL, and an Uplink, UL. The Downlink is a transmission from a “master” node such as a base station (eNB) to a “slave” node such as a User Equipment, UE. The Uplink is a transmission from the “slave” node to the “master node”. A downlink transmission may schedule an Uplink transmission on the radio link from the “slave” node.
Often the DL transmit power is a fixed power density according to a maximum power. And the UL transmit power is determined by the slave node, based on measurements of a downlink reference signal received from the master node. The aim is that all UL transmissions are received at the master node at more or less the same power.
In LTE, power control is applied per Uplink, UL, physical channel. For example, for the Physical Uplink Shared Data Channel PUSCH, the transmitted power by the UE in subframe i is determined by the formula:PPUSCH(i)=min{PCMAX,10 log10(MPUSCH(i))+PO_PUSCH(j)+α(j)·PL+ΔTF(i)+f(i)}where PCMAX(i) is the configured maximum UE transmit power, MPUSCH(i) is the number of resource block allocated for the UE, PO_PUSCH(j) is a parameter consisting of the sum of a cell-specific and a UE-specific part provided by higher layers, α is a cell-specific parameter configured by higher layers (also known as fractional pathloss compensation factor), PL is the downlink pathloss estimate calculated by the UE, ΔTF(i) is a UE-specific parameter provided by higher layers and f(i) is UE-specific correction term controlled by Transmit Power Control, TPC, commands sent in uplink grants on the Physical Downlink Control Channel, PDCCH. In later releases of the 3GPP specifications power control is slightly more complicated due to the support of multi-carrier where a UE can support multiple serving cells.
Antenna beamforming is a signal processing technique whereby, in a transmitting device comprising a plurality of transmit antennas, a signal to be transmitted is mapped to the plurality of transmit antennas so as to increase the power of the signal received at the receiving device. A transmitting device, for example a UE, may calculate a beamforming precoding vector for a transmission, based on measurements of reference signals received from the receiving device, for example using SVD (Single Value Decomposition).
At lower frequencies beamforming may not be necessary. However, at higher frequencies, such as those likely to be used in future wireless systems, beamforming is expected to be required.
The Applicant has appreciated that it may therefore be desirable to use antenna beamforming in UEs, including those which have long discontinuous transmission, DTX, and/or discontinuous reception, DTX inactive times. These times are effectively periods, in which the UE is in “sleep mode”, and so does not transmit and or receive transmissions. This enables low power consumption, which is advantageous for many for example machine type applications. However, these inactive periods make uplink beamforming difficult, since the decorrelation time of a beamforming precoding vector may be shorter than the DTX and/or DRX period, during which no reference signals are available for beamforming calculations. In current systems such as LTE, the DTX/DRX reference signal non-availability duration is a maximum of 5 ms. It is expected that in 5G, the DTX/DRX reference signal non-availability duration will be up to 100 ms.